Method for producing an aluminum-titanium-boron prealloy for use as a grain refiner

ABSTRACT

The invention relates to a method for producing a grain refiner on the basis of an aluminum-titanium-boron prealloy. According to the inventive method, starting materials that contain Ti and B are introduced into an aluminum melt while TiB 2  particles are formed, and the prealloy melt produced is allowed to solidify. The prealloy is set in motion at a temperature between the liquidus temperature (T L   Al3Ti ) of the Al 3 Ti phase and the solidus temperature (T S   V ) of the prealloy for a period (Δt d ) sufficient to disperse the TiB 2  particles in the melt. The melt is simultaneously cooled off at a first rate of cooling (v 1 ) so that the TiB 2  particles function as the nuclei for the Al 3 Ti phase that is formed below the liquidus temperature (T L   Al3Ti ) and the surface of the TiB 2  particles is at least partially covered by an Al 3 Ti coating. The prealloy is then cooled off to a temperature below the solidus temperature (T S   V ) of the prealloy at a second rate of cooling (v 2 ) that is higher than the first rate of cooling (v 1 ). The inventive method is especially useful in the production of grain refiners for the grain refinement of aluminum and aluminum alloys.

[0001] This invention concerns a process for the manufacture of a grain refinement medium on the basis of an aluminium-titanium-boron pre-alloy through the introduction of raw materials containing Ti and B into an aluminium melt under formation of TiB₂ particles and the solidifying of this pre-alloy melt.

[0002] EP-A-0396389 describes a process for the continuous manufacture of an Al—Ti—B grain refinement alloy, in which raw materials containing Ti and B are introduced into a reaction zone of an aluminium melt, whereby the melt is agitated in the reaction zone. A mixture of the formed alloy together with the reaction products is supplied continuously to a refining zone in which the slag with the reaction products is continuously collected on the surface of the melt and removed. The formed grain refinement alloy is continuously passed from the refining station to a casting station, in which the melt is continuously cast into a strand. The cast strand can either have the desired strand or wire width directly, or it can be worked through further treatment by way of rolling or extrusion molding into the desired grain refinement material.

[0003] The article by P. Schumacher et al, “New studies of nucleation mechanisms in aluminium alloys: implications for grain refinement practice”, Materials Science and Technology, May 1998, Vol. 14, pages 394 to 404, discloses a plausible theory for the order of events in the grain refinement of aluminium alloys by way of the addition of an Al—Ti—B pre-alloy of the composition AlTi5B1 for example. According to this theory, the best grain refinement results are obtained when the surface of the TiB₂ particles insoluble in the aluminium melt is at least partly covered with a layer of Al₃Ti phase. The nucleation of the α-aluminium phase takes place on the Al₃Ti layers, the effect of which increases with decreasing layer thickness.

[0004] The TiB₂ particles in the Al—Ti—B grain refinement media known today are highly susceptible to the formation of agglomerates. The result is a diminished effect of the grain refinement medium. Further disadvantages arise through agglomerates and inclusions which can lead to faults in the end product. Examples of this are grey lines, holes, material separation and stringers. In addition agglomerates occur preferably on low melt salts such as for example KF and NaCl and on oxide skins and can spread further as a result. Such agglomerates are “soft”, and can be forced through filters and as such reach the cast strand.

[0005] The invention is therefore based on the task of preparing a process for the manufacture of an Al—Ti—B grain refinement medium, with which the formation of agglomerates of TiB₂ particles can be largely prevented and existing agglomerates can be deagglomerated.

[0006] The task is resolved with a process of the type described initially, whereby the pre-alloy is set in motion between the liquidus temperature of the Al₃Ti phase and the solidus temperature of the pre-alloy for a sufficient time period for the dispersal of the TiB₂ particles in the melt, and at the same time the pre-alloy is cooled at a first cooling rate so that the TiB₂ particles act as nuclei for the Al₃Ti phase occurring below the liquidus temperature and the surface of the TiB₂ particles are at least partly covered with a coating of Al₃Ti, and in that the pre-alloy is then cooled below the solidus temperature of the alloy at a second cooling rate higher than the first cooling rate.

[0007] The term “motion of the melt” refers to all process steps intended to largely eliminate the formation of agglomerates of TiB₂ particles and deagglomerate existing agglomerates. These include among others mechanical stirring and vibration processes at high revolutions of the agitator and the production of cavitations, i.e. the formation of bubbles, the implosion of which causes shock waves which lead to the deagglomeration of agglomerated particles. Included in the latter process are for example the ultrasound treatment and vibration by means of a magneto-hydrodynamic resonator.

[0008] The grain refinement medium manufactured with the process in accordance with this invention causes, especially in the case of grain refinement of casting formats from aluminium alloys, an improved and more homogenous action of the grain refinement medium by way of a more homogenous distribution of the individual TiB₂ particles, a better coating of the TiB₂ particles with Al₃Ti phase and a reduction or dispersal of any salts and oxide inclusions remaining in the grain refinement medium.

[0009] Preferably, the pre-alloy will already be set in motion before the temperature falls below the liquidus temperature of the Al₃Ti phase.

[0010] The action of the grain refinement medium manufactured in accordance with the invention is shown in the fact that as a result of coating formed from a thin Al₃Ti layer, the individual TiB₂ particles, which are roughly 0.5 to 5 μm wide, show an excellent nucleation action and the particles act in isolation and not as agglomerates, so that a comparable grain refinement can be achieved with a considerably smaller amount of grain refinement medium than with a grain refinement medium according to the state of the art. In practice, this means that the grain refinement medium can be manufactured in a considerably more diluted form, which further reduces the tendency of the TiB₂ particles to form agglomerates.

[0011] Depending on the configuration of the device used for the execution of the process in accordance with the invention, it can occur that the temperature falls prematurely below the liquidus temperature of the Al₃Ti phase. This is above all the case if a pre-alloy with a high Ti content and correspondingly higher liquidus temperature is manufactured, or if an already solidified pre-alloy is used as the raw material. By reheating the melt above the liquidus temperature, Al₃Ti particles which are already separated can be returned completely to solution. This process takes typically 5 to 60 min, depending on the size of the Al₃Ti particles.

[0012] In an especially preferred process in accordance with this invention the movement of the melt occurs by way of sound, preferably ultrasound, where the melt is suitably exposed to sound frequencies of 50 Hz to 50 kHz, preferably at least 10 to 30 kHz.

[0013] The second cooling rate is preferably more than 1° C./sec, especially more than 5° C./sec, and notably preferably more than 10° C./sec.

[0014] The pre-alloy melt can be cast into any format. Preferable however is a strand suitably manufactured continuously by way of vertical or horizontal casting. This strand can either be cast already in the format of the rod or wire material desired as grain refinement medium or can be worked further by way of rolling or pressing into the rod or wire material. Vertically cast large strands especially are further worked by extrusion molding. The horizontal continuous casting of formats with relatively small diameter is preferred as this process allows continuous production. The horizontally extruded casting formats are further worked preferably through rolling to the desired rod or wire material.

[0015] A pre-alloy manufactured with the process in accordance with this invention has a composition, the total titanium content of which exceeds the stoichiometric ratio of TiB₂. A preferred pre-alloy contains titanium and boron in a ratio of 5:2 to 10:1. Although the process is suitable for the manufacture of pre-alloys with 0.15 to 20 w. % titanium and 0.01 to 4 w. % boron, it has proved favourable if the pre-alloy contains 0.3 to 5, preferably 0.5 to 2, w. % Ti and 0.02 to 1, preferably 0.05 to 0.5, w. % B.

[0016] The process in accordance with this invention is especially suitable for the manufacture of grain refinement media for the grain refinement of aluminium and aluminium alloys.

[0017] Further advantages, characteristics and features of the invention can be found in the following description of the preferred embodiment examples and from the drawing; this shows diagrammatically in:

[0018]FIG. 1 a section from the Al—Ti equilibrium diagram;

[0019]FIG. 2 a cross-section through an installation for the manufacture of a Al—Ti—B pre-alloy.

[0020] The Al—Ti equilibrium diagram shown in FIG. 1 represents the schematic process sequence for the manufacture of an Al—Ti—B pre-alloy for the grain refinement of aluminium alloys.

[0021] A pre-alloy manufactured according to the process in accordance with this invention with a composition corresponding to AlTi0.7B0.1, which contains roughly 0.5% titanium not bonded to boron, has a starting temperature of roughly 840° C. and thus lies above the liquidus temperature T^(L) _(Al3Ti) of the Al₃Ti phase for this alloy composition, roughly 800° C. The graphic depiction A of the alloy phase to the left of the 0.5% Ti line shows the events involved in the preparation of an already solidified pre-alloy, and depiction B to the right of the 0.5% Ti line shows the events during the solidification of the pre-alloy.

[0022] The pre-alloy melt contains TiB₂ particles in partly agglomerated form. Before the temperature falls below the liquidus temperature T^(L) _(Al3Ti) and until just before it falls below the solidus temperature T^(S) _(V) of the pre-alloy of the Al₃Ti phase, the partly agglomerated TiB₂ particles are deagglomerated and homogeneously distributed by a violent melt movement by means of an ultrasonic treatment at a frequency of for example 25 kHz. Through the simultaneous controlled cooling with a first cooling rate v₁ of e.g. 0.5° C./sec, a thin layer of Al₃Ti phase is deposited on the parallel surfaces of the TiB₂ particles and simultaneously the formation of coarse grained Al₃Ti particles is prevented. The ensuing severe cooling below the solidus temperature T^(S) _(V) of the pre-alloy at a second cooling rate v₂ higher than the first cooling rate v₁ of for example 10° C./sec, ensures that the Al₃Ti layer on the TiB₂ particles does not detach fully and also no further formation or coarsening of Al₃Ti particles occurs.

[0023] A plant 10 shown in FIG. 2 for the manufacture of an Al—Ti—B pre-alloy for the grain refinement of aluminium alloys comprises a reaction vessel 12 with an inlet channel 14 in its upper area and an outlet channel 16 in its lower area. The reaction vessel 12 is surrounded by an induction motor 18 as an electromagnetic stirring device with which the aluminium melt 20 in the reaction vessel 12 is agitated violently under formation of a vortex 22. Salts containing Ti and B such as for example K₂TiF₆ and KBF₄ are supplied in the direction of arrow 24 to the vortex 22 which mixes the salts into the aluminium melt 20.

[0024] The aluminium melt 20 with the reaction products is then passed through the outlet channel 16 by way of a further inlet channel 26 into the upper area of a further treatment vessel 28. A further electromagnetic stirring device 30 in the lower area of the further treatment vessel 28 leads to a lower turbulent zone 32 and an upper settling zone 34. The slag 36 formed through the reaction products is removed by way of the removal opening 38 from the further treatment vessel 28.

[0025] The cleaned aluminium melt 20 with the titanium and boron elements contained therein is added as a pre-alloy in the lower area of the further treatment vessel 28 by way of a pouring channel 38 to a mould, not shown in the drawing, of a horizontal casting machine. In the area of the pouring channel 38 are arranged two ultrasound emitters 40, 42, the sonotrodes 44, 46 of which dip into the melt. An induction heater 48 arranged below the pouring channel 38 serves to heat the melt should its temperature fall below the liquidus temperature T^(L) _(Al3Ti) of the Al₃Ti phase as the melt runs from the further treatment vessel 28 into the launder 38. 

1. Process for the manufacture of a grain refinement medium on the basis of an aluminium-titanium-boron pre-alloy through the introduction of raw materials containing Ti and B into an aluminium melt under formation of TiB2 particles and a solidifying of this pre-alloy melt, characterised in that the pre-alloy is set in motion between the liquidus temperature (T^(L) _(Al3Ti)) of the Al₃Ti phase and the solidus temperature (T^(S) _(V)) of the pre-alloy for a sufficient time period (Δt_(d)) for the dispersal of TiB₂ particles in the melt, and at the same time the pre-alloy is cooled at a first cooling rate (v₁) so that the TiB₂ particles serve as nuclei for the Al₃Ti phase occurring below the liquidus temperature (T^(L) _(Al3Ti)) and the surface of the TiB₂ particles are at least partly covered with a coating of Al₃Ti, and in that the pre-alloy is then cooled below the solidus temperature (T^(S) _(V)) of the pre-alloy at a cooling rate (v₂) higher than the first cooling rate (v₁).
 2. Process according to claim 1, characterised in that the pre-alloy is set in motion before the temperature falls below the liquidus temperature (T^(L) _(Al3Ti)) of the Al₃Ti phase.
 3. Process according to claim 1 or 2, characterised in that if the temperature prematurely falls below the liquidus temperature (T^(L) _(Al3Ti)) or in the event of pre-solidifying of the pre-alloy, already separated Al₃Ti particles in the melt are heated to above the liquidus temperature (T^(L) _(Al3Ti)) until fully dissolved.
 4. Process according to one of claims 1 to 3, characterised in that the motion of the melt is achieved through stirring or vibration of the melt.
 5. Process according to one of claim 1 to 4, characterised in that the motion of the melt is achieved through cavitation.
 6. Process according to claim 5, characterised in that the motion of the melt is achieved by means of sound, especially by means of ultrasonic.
 7. Process according to claim 6, characterised in that the melt is exposed to sound at a frequency of 50 Hz to 50 kHz, preferably 10 to 30 kHz.
 8. Process according to claim 5, characterised in that the motion of the melt is achieved by means of a magneto-hydrodynamic resonator.
 9. Process according to one of claims 1 to 8, characterised in that the second cooling rate (v₂) is greater than 1° C./sec, preferably greater than 2° C./sec, in particular greater than 5° C./sec.
 10. Process according to one of claims 1 to 9, characterised in that the pre-alloy melt is cast into a strand.
 11. Process according to claim 10, characterised in that a continuous strand is manufactured, preferably through horizontal continuous casting.
 12. Process according to claim 10 or 11, characterised in that the strand is drawn further into grain refinement strands or wires.
 13. Process according to one of claims 1 to 12, characterised in that the pre-alloy has a composition, the total titanium content of which exceeds the stoichiometric ratio of TiB₂.
 14. Process according to one of claims 1 to 13, characterised in that the pre-alloy contains Ti and B in a weight ratio of 5:2 to 10:1.
 15. Process according to one of claims 1 to 14, characterised in that the pre-alloy contains 0.05 to 20, preferably 0.1 to 5, in particular 0.5 to 2, w. % Ti and 0.01 to 4, preferably 0.02 to 1, especially 0.05 to 0.5, w. % B.
 16. Use of the process according to onr of claims 1 to 15 for the manufacture of a grain refinement medium for the grain refinement of aluminium and aluminium alloys. 